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I don't bet, but if I did, I'd bet that it is a male robin.
The general size and shape is right for a robin.
Reading Kluut's comment - I should have thought of their tendency to fight: it's very likely the cause of the de-feathering.
Possibly not sick - it seems "bright-eyed" and to be eating, and definitely not moulting - no bird moults whole areas of feathers at once, apart from flight feathers in some species, and not at this time of year. However, they fight like cat and dog at this time of year, which is a more likely cause and hence my suggestion that it is a male.
I've seen tatty robins before but it is incredibly unusual for one to lose ALL of the feathers from one area. The fighting robins theory is a good one, but the daft bird must have kept very still to allow its rival to make such a thorough job of it. I wonder if this is more likely to be the result of the bird getting its head stuck in something, garden netting perhaps?
Although the orangy-red breast is entirely missing, I have little doubt that this is a Robin.
As for the reason for it's baldness, there have been a number of suggestions why bald birds like this sometimes occur including malnutrition, disease, mites and abnormal moult (+ Kluuts reasonable suggestion that this one may be the result of a lost fight). As far as I know, no conclusive evidence has been found for any of these suggestions yet though!
Since Robins have been shown to attack stuffed models, and it is the red breast that apaprently triggers the attacks, perhaps a rival could have 'laid into' this one after it had somehow been temporarily stunned (eg after flying into a window), and continued to attack until all the red feathers were gone!? I guess that we'll never know.
Nick Baker mentioned this picture on twitter a couple of weeks ago:-
It's a blackbird with a bald head; similar to the bird here.
Nick said that this individual was in its second year of breeding. Obviously Nick has seen the individual before and the feather loss was permanent in that case.
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I think it could be an unnaturally large nestling that has not fully developed it's feathers.
Nestlings inhabit nests - I think you mean fledgling :-).
Fledgling robins are spotted, with no red feathers (to avoid agression from their parents), and will not be around for a few weeks yet, and are fully feathered when they do fledge.
Lat/Lng: 51.5, -3.4
OS grid ref: ST0785